Aqueous colloidal dispersions of polymer particles having diameters in the range of less than one micron are well-known in the art and are referred to as latexes. Waterborne latexes have found ubiquitous applications in such areas as paint formulations, adhesives, and sizing agents. Latexes are prepared via emulsion polymerization, whereby an emulsion of monomers, surfactants, polymerization initiators, and other additives is subjected to polymerization conditions and the resultant latex is collected. Emulsion polymerization is advantageous for a number of reasons: no undesirable organic solvents are used; the aqueous medium acts to effectively control the heat of the polymerization and provides convenient control over the viscosity of the resultant colloid; a high-molecular-weight polymer is produced; and the latex can be coated directly onto a web or other substrate.
A two-step emulsion polymerization technique is a well-known method of preparing so-called "core-shell" latex polymer systems. Core-shell particles have an inner "core" formed as a first step of the polymerization which can comprise homopolymeric or copolymeric systems, and a "shell" formed as a second step of the polymerization which can comprise homopolymeric or copolymeric systems which are, generally, different from that of the core. In practice, more than one such layer can be built upon a core. Such systems are useful where two or more distinct sets of properties of the final polymer composition are desired.
A core-shell latex comprising a core of ethyl acrylate/methyl methacrylate copolymer surrounded by a shell of a styrene/(meth)acrylate copolymer is described in U.S. Pat. No. 4,894,397. An "inverted core-shell" polymerization process is employed, wherein the first stage the shell and the second stage polymer is incorporated therein to become the core. Latexes disclosed therein are used in coatings, inks, and films.
A core-shell latex for textile treatment comprising (meth)acrylate copolymeric species for both the core and shell is described in U.S. Pat. No. 4,351,875. Both the shell and the core are required to contain a copolymeric "latent crosslinking monomer," generally a (meth)acrylic amide or (meth)acrylic N-alkylolamide which are known to impart self-curing characteristics to compositions containing them. The additional hydroxyl functionality of these monomers is required in order to bind the latex to the texile surface. In addition, the latex core is required to contain a polyfunctional free-radically polymerizable crosslinking agent.
U.S. Pat. No. 5,021,469 discloses a binder for waterbased gloss paints comprising a core-shell emulsion polymer wherein the core is a hard, high T.sub.g composition and the shell is a softer copolymer of at least three components which contains, inter alia, a nitrogen-containing adhesive monomer, one example of which is N-vinylpyrrolidone, although it is not the preferred nitrogenous constituent. The shell is also required to contain an ethylenically-unsaturated carboxylic acid monomer.
An aqueous dispersion of core-shell latex particles for formulation of paints is described in French Patent Application No. FR 2,557,574, in which the core comprises a copolymer of (meth)acrylate and hydrophilic monomers and the shell comprises the same monomers but is constructed such that 50-90% of the hydrophilic monomers reside in the shell, although the shell comprises only 1-15% of the total weight of the latex. Hydrophilic monomers used therein include N-vinyl pyrrolidone, (meth)acrylamide and aminoalkyl (meth)acrylates, as well as other non-nitrogenous monomers. A coating material comprising the latex is described in European Patent Application No. EP 207,854.
Copending U.S. patent application Ser. No. 08/024,636, filed Jan. 27, 1993, assigned to the assignee of the present invention, discloses certain solution-polymerized acrylamide (co)polymers that are useful as binders for microsphere adhesives. The binder materials are not latexes and are coated from organic solvent(s).